Conventionally, noise reduction of motors has been attempted, for example, by reducing the operating noise of motors. Generally, motors are designed in an optimal manner to reduce noise by selecting the materials and shapes of components that affect the resonant frequencies of the components and the number of poles of the motor. However, it is difficult and troublesome to reduce noise of motors by optimizing only the mechanical features of the components.
For example, Japanese Laid-Open Patent Publication No. 2007-1366 discloses a technology for reducing the operating noise of a motor through electric control, namely, by adjusting the direct-current voltage supplied to the motor.
Minute torque fluctuations in one revolution of a motor is one of the causes of motor vibration. However, the above publication does not provides no concrete solution against such minute torque fluctuations. Thus, it is considered that motor vibration caused by minute torque fluctuations is not sufficiently reduced. There is room for improvement in technologies for reducing noise.